1. Field of the Invention
The present invention relates to a piezoelectric vibrator, a manufacturing method of the piezoelectric vibrator, and an oscillator, an electronic device, and a radio-controlled timepiece.
2. Description of the Related Art
Recently, a piezoelectric vibrator using a piezoelectric vibrating reed made of a piezoelectric material such as quartz has been used in cellular phones and portable information terminals as the time source, the timing source of a control signal, a reference signal source, and the like. As the piezoelectric vibrating reed, for example, a tuning-fork type piezoelectric vibrating reed having a pair of vibrating arms is used.
As the piezoelectric vibrator of this type, an SMD (Surface Mount Device)-type piezoelectric vibrator is known. As an example of the SMD-type piezoelectric vibrator, there is proposed one in which a package is formed by a base board and a lid board, and a piezoelectric vibrating reed is accommodated in a cavity formed inside the package.
Meanwhile, in general piezoelectric vibrators, it is preferable to suppress an equivalent resistance value (effective resistance value Re) to a low value. Since a piezoelectric vibrator having a low equivalent resistance value is capable of vibrating a piezoelectric vibrating reed with a low power, a piezoelectric vibrator having high energy efficiency can be achieved.
As a typical method of suppressing the equivalent resistance value, there is known a method of creating a near-perfect vacuum in the sealed cavity of the piezoelectric vibrating reed so as to decrease a series resonance resistance value (R1) which is proportional to the equivalent resistance value. Moreover, as a method of creating a near-perfect vacuum in the cavity, JP-A-2006-86585 discloses a method (gettering method) of accommodating a gettering material in the cavity and activating the gettering material with laser irradiation from the outside. According to this method, since gas (for example, oxygen) surrounding the gettering material can be absorbed by the activated gettering material, it is possible to create a near-perfect vacuum in the cavity.
In addition, in a typical manufacturing method of the piezoelectric vibrator, a rough tuning step of tuning the frequency of the piezoelectric vibrating reed is performed before the gettering so that the frequency of the piezoelectric vibrating reed falls near a target frequency (nominal frequency). After the series resonance resistance value is adjusted by the gettering, a fine tuning step of tuning the frequency of the piezoelectric vibrating reed is performed so that the frequency of the piezoelectric vibrating reed finally falls within the range of the nominal frequency.
However, the piezoelectric vibrator of the related art has still the following problems.
That is, when the gettering material is activated by laser irradiation or the like, it is highly likely that materials constituting the evaporated gettering material will be scattered to be deposited onto the piezoelectric vibrating reed. When the constituent materials of the gettering material are deposited onto the piezoelectric vibrating reed, there is a problem in that the frequency of the piezoelectric vibrating reed is changed. The frequency change appears differently depending on the position where the gettering material is deposited. For example, in the case of a tuning-fork type piezoelectric vibrating reed, the frequency tends to decrease when the gettering material is deposited onto the tip end of its vibrating arm, and the frequency tends to increase when the gettering material is deposited onto the base end of its vibrating arm.
Moreover, when the frequency of the piezoelectric vibrating reed is changed before and after the gettering step, the frequency having fallen near the nominal frequency during the rough tuning step is changed before the fine tuning step, it may be difficult to make the frequency fall within the range of the nominal frequency by the fine tuning step.